You can add/change it at any time by going to 'Profile' on the navbar and then clicking on Avatar next to Signature.

You will then be given the option to upload an avatar from your computer, Upload an avatar from a URL, Link to an off-site avatar or select an avatar from our gallery. If you've got a Direct link to your avatar, paste it into either the Link to an off-site avatar box or the upload an avatar from a URL box and press Save. To choose an avatar from your computer, click on Browse and select it from your documents.